Beth,
Check in with Efficon and asked for some info on their newsfeed generator. Meantime, I looked at their site and was able to subscribe to a page that has articles on it, and hopefully it may be using the same method to generate a newsfeed as your module. As I inspected it, turns out it's not all that mystical. Here's some 'raw' code from that feed (with apologies to Efficon)
<rss version="2.0">
The above indicates that you should be able to parse this feed with an xsl file that uses RSS v2.0, but looking at the feed, I really don't see much in the way of RSS v2.0 being used. So, the only thing you may have to do is disable output escaping on the <description> element. One thing though, I am not sure if the feed I'm looking at is from the Articles module or not.
Now, if you could 'syndicate' your Articles feed, I may be able to subscribe to it to troubleshoot further. The issue here is not really due to a Core DNN module but a 3rd party module's syndication method. Like many other developers there is a nice thought about upgrading the newsfeed generated, but leaves out the xsl to properly use it. Now, this isn't really a slam of Efficon, as the modules from them are pretty nice. It's probably something that went without saying that you would need a custom xsl to use it. At least that's what I initially see. Of course, I don't actually use that module, so until I see an example of the raw newsfeed generated by it, I'm really just guessing.
I've contacted Efficon offering to provide a custom xsl for them, and hope I get a positive response. I don't do this for a living, though I'd like it. I really contribute in the ways I can due to my history of using DNN and as a former Core Team memeber, with hopefully many friends out in the DNN global community, I think the help is appreciated.
Anyway, if you could syndicate that content on your site, I may be able to help you more.
Cheers